1. Field of Invention Application
The present invention relates generally to a platen knob and more particularly to a platen knob suitable for use in electronic apparatuses such as an impact dot printer.
2. Description of the Related Art
The proportion of impact dot printers to printers is large and the impact dot printers are popular in offices. However, a printing noise by the impact dot printers has recently given rise to a trouble.
A prior-art impact dot printer will be described hereinafter. FIG. 8 is a perspective view of the prior-art impact dot printer. The printer is generally indicated at 50. An upper cabinet of the printer is indicated at 51. The printer 50 comprises a platen knob 52.
FIG. 9a illustrates a main part of the interior of the printer 50. A cylindrical platen 53 is journalled on a framework (not shown) within the upper cabinet 51. A paper feeder (not shown) feeds a printing paper 54 to the platen 53. The printing paper 54 is fitted on the cylindrical surface of the platen 53. A carriage guiding shaft 60 is supported on the framework and extends in parallel to the platen 53. A carriage 61 carrying an impact dot printer head 55 is slidably mounted on the carriage guiding shaft 60. The printer head 55 has a predetermined distance from the cylindrical surface of the platen 53. The carriage 61 carries an ink ribbon 56 covering the front edge 55a of the printer head 55 and is fastened to a drive belt 62. The drive belt 62 extends between a drive pulley 63 and a driven pulley 64 spaced from each other so that the carriage 61 can reciprocally travel across the length of the platen 53. The drive pulley 63 is mounted on the shaft 65a of a spacing motor 65. The torque of the spacing motor 65 is sequentially transmitted to the drive pulley 63, the drive belt 62 and the carriage 61 to move the carriage 61 in a spacing direction. An interior mechanism (not shown) of the carriage 61 transforms a movement of the carriage 61 along the carriage guiding shaft 60 to a rotation of an ink ribbon take-up shaft (not shown) to take up the ink ribbon 56. One end 53b of a shaft of the platen 53 has a spur gear 68 mounted thereon. The shaft 66a of a line feed motor 66 has a pinion 67 mounted thereon and meshing with the spur gear 68. The torque of the line feed motor 66 is sequentially transmitted to the pinion 67, the spur gear 68 and the platen 53 to rotate the platen 53.
The respective printer head 55, spacing motor 65 and line feed motor 66 are electrically connected to a control board 73 through a lead wire 70 from the printer head 55, a lead wire 71 from the spacing motor 65 and a lead wire 72 from the line feed motor 66. The control board 73 comprises a printer control device 74 performing a printing control of the printer head 55, a spacing control of the carriage 61 and a line feed control of the printing paper 54.
FIG. 9b is a sectional view of the knob 52 taken along the line Y--Y in FIG. 9a. The body of the knob 52 is essentially a bottomed hollow round cylinder. A central part of the interior surface of bottom wall 52c of the knob 52 has a fitting boss 52a extending inward of the printer 50. The fitting boss 52a is a double cylinder comprising a fitting inner cylinder 52d, a reinforcing outer cylinder 52e, and ribs (not shown) extending radially from the inner cylinder 52d to the outer cylinder 52e. The fitting boss 52a is fitted on an external end 53a of the shaft of the platen 53.
In operation, manually rotating the knob 52 rotates the shaft of the platen 53 and the platen 53. The manual rotation of the knob 52 also directly causes the paper feeder to feed the printing paper 54 in the direction of rotation of the platen 53. Thus, the printing paper 54 is positioned in the printer 50.
Then, supplying printing instructions to the printer 50 starts a series of operations of the printer 50. The printer 50 line feeds the printing paper 54 to a line of letters to be printed. The printer control device 74 controls the line feed motor 66 to sequentially transmit a torque to the pinion 67, the spur gear 68 and the platen 53 and finally rotate the platen 53. Thus, the paper feeder rotating together with the platen 53 line feeds the printing paper 54 to the line of letters to be printed.
The printer head 55 is then spacing-moved along the carriage guiding shaft 60 to the position of a column of letter to be printed. The printer control device 74 controls the spacing motor 65 to rotate the drive pulley 63 and move the drive belt 62 and the carriage 61 together along the carriage guiding shaft 60. The carriage 61, the printer head 55 and the ink ribbon 56 together travel along the carriage guiding shaft 60 to follow the spacing-movement.
The printer 50 then starts printing letters or the like. The printer control device 74 processes printing data of printing instructions to dot matrices constituting letters and supplies drive signals required for printing to the printer head 55. Thereby, predetermined dot pins (not shown) of the printer head 55 impact the printing paper 54 fitting on the platen 53 via the ink ribbon 56. The dot pins transfer an ink impregnated in the ink ribbon 56 to the printing paper 54 to print one column of a dot matrix constituting a letter on the printing paper 54.
The printer control device 74 then spacing-controls the printer head 55 and the ink ribbon 56 to move to a next column of the dot matrix. The printer control device 74 controls the printer head 55 to print the next column of the dot matrix on the printing paper 54. The printer 50 repeats the cycle of the above-described operations to print letters and further lines of letters. In this case, the ink ribbon 56 is taken up as the carriage 61 moves along the carriage guiding shaft 60, so that the impacting surfaces of the dot pins always receive a fresh part of the ink ribbon 56. When the printer 50 prints a next line of letters on the printing paper 54, the printer control device 74 controls the line feed motor 66 to rotate the platen 53 by an angle corresponding to one vertical spacing between adjacent lines of letters. The paper feeder feeds the printing paper 54 by the one vertical spacing so that the printer head 55 prints a new line of letters. Thus, the printer control device 74 repeats the cycle of the line feed control, the spacing control and the impact control to print on the overall sight of the printing paper 54.
When the printer having the above-described structure operates, the impacts by the dot pins finely vibrates the platen 53. This fine vibration propagates from the shaft of the platen 53 to the fitting boss 52 of the knob 52a. Since a component of the fine vibration and the natural vibration of the knob 52 depending on the shape of the bottom wall 52c of the knob 52 resonate to each other, the bottom wall 52c of the knob 52 constitutes a source of noise. In addition, a cavity defined between the fitting boss 52a, the outermost sidewall 52b and the bottom wall 52c of the knob 52 resonates to the fine vibration to increase noise. The cavity also serves as a passageway through which the impact sounds of the dot pins from the cylindrical surface of the platen 53 leak out of the upper cabinet 51. Therefore, measures for fitting the knob 52 on the end 53a of the shaft of the platen 53 via an elastic element have been taken. However, these measures cannot achieve a sufficient noise reduction.